1. Field of the Invention
The present invention relates to a method pertaining to the fine-grinding of minerals and similar materials down to a particle size in which the finely ground material can be used suitably as a filler. The present invention also relates to a mill arrangement for use when carrying out the method.
2. Background Information
Minerals and similar materials intended for use as a filler in the production of different products, for example, in the manufacture of paper, plastics, paints, coatings, adhesive products and sealing materials, must have an average particle size which lies at least beneath 45 .mu.m (97%). Furthermore, it is necessary that the material has a specific surface area corresponding to a Blaine-number greater than 400 m.sup.2 /kg. In the majority of cases, an average particle size smaller than 10 .mu.m is required, for instance, when the material is used as a filler in paper and paints, while certain other applications require a still finer particle size, so-called ultra fine particles having an average particle size or grain size of &lt;2 .mu.m, for example, when the material is used as a filler in paper sizing coatings.
In certain cases, the filler material used for these purposes may comprise a precipitate which already has the desired particle size, or a particle size which lies close to the desired particle size, although filler materials are normally produced by a grinding process that includes a fine-grinding stage in which minerals or similar natural materials are ground to a desired particle fineness. Standard materials from which fillers are produced include different carbonate materials, such as lime stone or dolomite, different sulphate materials, such as gypsum, and silicon-based material, for example, clays such as kaolin. Fine-ground products of this kind cannot be produced readily by wet grinding processes, such processes being those normally applied for grinding materials down to desired fineness, since a wet-ground product needs to be subsequently dried. The fine material tends to lump together during this drying process and the resultant agglomerates need to be broken down in a further grinding process. The capital investment required herefor renders the wet-grinding alternative prohibitive in the majority of cases. In consequence, it is necessary to use a dry grinding process which, in the majority of cases, implies the use of a mill which operates with an agitated grinding medium, although it should be possible to use other grinding methods, at least in conjunction with smaller quantities of material, for instance batch wise grinding methods using steel or ceramic grinding bodies. The inventive method, however, is discussed below primarily with reference to an agitated grinding medium.
The technique of grinding down material with the aid of an agitated medium (Stirred Ball Milling) has been known to the art for almost 60 years. The technique had its industrial breakthrough in 1948, in conjunction with pigment grinding in the paint and lacquer industry. The technique has been developed progressively during recent years and has obtained increased application. As a result, many different types of grinding mills that use an agitated medium have been proposed, as is evident, for instance, from an article published in International Journal of Mineral Processing, 22 (1988), pages 431-444. One of these mills is equipped with pin agitator rotors, by means of which the requisite grinding energy is introduced by forced displacement of the grinding medium. Because the mill is able to grind material rapidly down to extremely fine-grain sizes, normally within the range of 1-10 .mu.m, the technique of grinding with the aid of an agitated medium has been applied to an increasing extent for various types of material. For example, fine grinding of this nature is applied in the production of fine-grain products within the fields of paint and lacquer technology, pharmacology, electronics, agrochemistry, food-stuffs, biotechnology, rubber, coal and energy. Examples of this latter case include coal-oil-mixtures and coal-water-suspensions. The technique of grinding with an agitated medium is now also being applied within the mineral processing field. Examples of such application include the grinding of limestone, kaolin, gypsum, aluminium hydroxide and the manufacture of paper fillers and paper coating materials, as beforementioned.
The results of experiments and tests carried out in recent years have shown that when grinding with an agitated grinding medium, the fineness of the ground material is dependent solely on the specific energy input, which can be expressed in kWh/tonne of material ground. Furthermore, it is found that the advantages afforded by this grinding technique over the alternative techniques are greatly enhanced with increasing fineness of the ground material, in other words grinding with the aid of an agitated grinding medium becomes more attractive with the desired fineness of the end product. Thus, a finer end product requires a higher specific energy input, i.e. a higher specific power input and/or longer grinding time. Obviously, it is preferred primarily to try with a higher power input, so as not to influence the productivity of the mills concerned negatively. Grinding times of 6-8 hours, which have been suggested, for instance, in conjunction with the grinding of pyrites in South Africa, are naturally not so attractive, although in many cases necessary, since a higher power input would place even greater demands on the ability of the mill to withstand a harsh environment, particularly when grinding harder materials.
A suitable mill for grinding material down to extremely fine-grain products with high power inputs is described in our earlier publication EP-A-0 451 121, while a suitable continuous grinding method for application in such mills is described in SE-A-9100884-7 (EP-A-0506638).
One serious problem experienced when finely grinding materials in a dry state resides in the occurrence of a cladding or blocking phenomenon, the actual cause of which cannot be established precisely, but which is accentuated with the fineness of the grain sizes to be produced. This phenomenon is probably caused by newly formed fine grains baking together, as a result of a combination of different physical forces, for instance surface phenomena, van der Waals forces and the formation of condensate.
One method of attempting to counteract the aforesaid problem involves the addition of a liquid dispersant to the material being ground. The primary drawbacks associated with the use of a dispersant are, of course, the costs of the chemicals used and the unavoidable contamination of the finished product. The demands placed commercially on the quality of certain fine grain products are so strict as to render a product which is contaminated with a dispersant or reaction products of such dispersant totally unacceptable. Consequently, these products must be finely ground with the utmost of care, therewith inhibiting productivity, partly with the intention of attempting to minimize cladding and partly because of the actual cladding phenomenon itself.